Internal combustion engine fuel control system

Abstract

An internal combustion engine fuel ignition system has feedback from an exhaust gas composition transducer which operates effectively only at normal exhaust temperatures, the transducer having a very high impedance at low temperatures. The feedback circuit includes a comparator stage and an integrator stage integrating the output of the comparator stage which compares the transducer output with a reference voltage. A feedback resistor is connected from the integrator stage output to the comparator stage input, so that, at low temperature the integrator stage output stabilises at a predetermined level. Only when the transducer warms up so that its impedance becomes significantly less than that of the feedback resistor does the exhaust feedback control loop become effective.

What is claimed is: 
     
       1. An internal combustion engine fuel control system comprising an exhaust gas composition sensing device having a high output impedance at low temperature, but producing when at operating temperature, a voltage signal related to the exhaust gas composition and a fuel control circuit to which said sensing device is connected so that variations in the voltage signal therefrom vary the fuel flow to the engine so as to cause the voltage signal to approach a desired level, said fuel control circuit including an input stage connected to the sensor and to a reference voltage source such that the output of said input stage is dependent on the relative levels of the signal voltage and the reference voltage, an integrator stage connected to the output of the input stage and producing an output which changes relatively slowly as a result of changes in the output of the input stage, the output of the integrator stage determining the rate at which fuel is supplied to the engine and a high impedance negative feedback circuit connecting the output of the integrator stage to an input terminal of the input stage and including a high impedance feedback path of impedance less than that of the sensing device at low temperature and higher than that of the sensing device at operating temperature, said feedback circuit causing the output of the integrator to approach a desired value when the sensing device is at low temperature. 
     
     
       2. A control system as claimed in claim 1 in which the input stage is a voltage comparator and the integrator stage is an operational amplifier integrator. 
     
     
       3. A control system as claimed in claim 2 in which the feedback circuit comprises a resistive potential divider connected between the operational amplifier output and an earth terminal and arranged to establish a voltage equal to the reference voltage when the operational amplifier output is at its desired value, said high impedance path being a high impedance resistor connected between said potential divider and an input terminal of the voltage comparator. 
     
     
       4. A control system as claimed in claim 2 or claim 3 further comprising a switch device connected between the comparator output and the integrator input and controlled so as to be conductive periodically at a frequency proportional to engine speed and for a fixed duration at each operation. 
     
     
       5. A control system as claimed in claim 1 further comprising an override switch connected in parallel with said high impedance feedback path. 
     
     
       6. A control system as claimed in claim 1 in which said input stage is constituted by a voltage comparator and said integrator stage is constituted by a multi-bit binary up/down counter, said voltage comparator output being connected to an up/down control terminal of the counter, there being also provided means for producing pulses at a frequency proportional to the engine speed connected to a clock input terminal of the counter. 
     
     
       7. A control system as claimed in claim 6 in which the feedback circuit is connected to a most significant bit output of the counter.